This invention relates generally to diesel engines and, more particularly, to fuel injection systems for diesel engines.
In a diesel engine, liquid fuel is injected into a plurality of engine cylinders full of compressed air at high temperature. The fuel is broken up into droplets, which evaporate and mix with the air in the cylinders to form a flammable mixture. The fuel efficiency and exhaust emissions of diesel engines are dependent upon the fuel injection timing and atomization. This is particularly true for quiescent type medium speed heavy-duty diesel engines where the cylinder air intake swirling is light, such as locomotive or marine type engines with relatively large displacement volumes.
For various reasons, including reducing exhaust emission of nitrogen oxides (NOX), it is sometimes desirable to retard the fuel injection timing of a medium speed diesel engine, i.e., retard the start of the fuel injection duration relative to conventional fuel injection start timing in an engine piston cycle. However, retarding the fuel injection timing increases untimely and/or incomplete combustion in the engine cylinders. Untimely combustion compromises engine efficiency and incomplete combustion increases exhaust emissions, including carbon monoxide (CO), particulate matters (PM) and smoke. Untimely and incomplete combustion can also have adverse effects on other engine components, such as turbochargers that derive energy from the exhaust gases. Untimely combustion increases the temperature of exhaust gases, which can lead to turbocharger overspeed and damage.
Accordingly, it would be desirable to provide a medium speed diesel engine that avoids performance deterioration at retarded fuel injection timings.
In an exemplary embodiment of the invention, a fuel injection system for a medium speed diesel engine is provided that increases fuel injection pressure and reduces a fuel injection duration to enhance engine efficiency and to reduce exhaust emissions of medium speed diesel engines operated at retarded fuel injection timing. The reduced fuel injection duration advances fuel injection duration ending time to an earlier point in the piston cycle, and the increase in fuel injection pressure improves the atomization of fuel. Consequently, combustion in the engine is improved.
The fuel injection system includes a fuel cam having a cam surface shaped to increase the cam lift velocity of a fuel injection system, thereby increasing fuel injection pressure, reducing fuel injection duration, and improving fuel atomization. A cam roller contacts the surface of the fuel cam and actuates a fuel injection pump plunger to control the fuel injection rate into the engine cylinders. The cam is rotated by a cam shaft about a rotational axis, and the shape of the cam causes the roller cam, and hence the fuel injection pump plunger, to move radially toward and away from the cam shaft.
Specifically, the cam surface includes a plunger return segment and a plunger advance segment. The plunger advance segment has an increasing radius so that when the cam roller contacts the plunger advance segment, the plunger is advanced into the fuel injection pump and forces fuel to be injected from a pump chamber into the engine cylinders. The plunger return segment has a decreasing radius so that the plunger is withdrawn from the fuel injection pump and draws fuel into the pump chamber. The plunger advance and return segments are oriented in a phase relationship with the compression stroke top-dead-center position so that plunger advance segment accommodates the retarded fuel injection timing and engine brake efficiency and performance are optimized.
The plunger advance segment increases rapidly in radius as the cam is rotated, and the plunger return segment decreases in radius relatively slowly. The rapid rise in the plunger advance segment radius increases the cam lift velocity relative to conventional cams. Increasing the cam lift velocity increases the fuel injection pressure, which improves atomization of fuel in the cylinders. The increased cam lift velocity also increases the rate of fuel injection, which reduces fuel injection duration and realizes an earlier, or advanced, fuel injection ending time. Consequently, the combustion of fuel in the cylinders is improved and untimely combustion in the engine cylinders is reduced. Thus the performance deterioration in engine efficiency and exhaust emissions due to retarded fuel injection timing are minimized.